Line balance for loaded telephone circuits



June 1930- A. c. BARTLETT 1,767,199

LINE-BALANCE FOR LOADED TELEPHONE CIRCUITS Filed March 30, 1925 INVENTOR. ALBERT CHARLES BARTLETT ATTY- 7 mined in Patented June 24, 1930 UNITED STATES PATENT! OFFICE ALBERT CHARLES BARTLETT, OF WATFORD, ENGLAND, ASSIGNOR TO THE GENERAL ELECTRIC COMPANY LIMITED, OF

LON, ENGLAND LINE BALANCE FOR LOADED TELEPHONE cmourrs Application filed March 30, 1925, Serial No.

The object of the invention is to provide improved line balances the impedance of which can be made to simulate closely over the range of telephonic frequencies the im pedance of a loaded cable which is terminated at half loading coil or over a considerable range of fractional coil terminations.

In the accompanying drawing, Fig. 1 illustrates one form of the invention andFig. 2 illustrates a modification.

A line balance according to the present invention consists of a new arrangement of elements having inductance, sistance, the values of which may be deterterms of the constants of the line to be balanced, 'as hereinafter more fully deance the cable scribed. 7

Although the line balance described in this specification, is primarily a mid-coil termination balance yet it can be adapted to balover a considerable range of fractional coil terminations.

Let the impedance of a loaded cable termipated at a half loading coil be considered and .Z=Impedance of an infinite periodically loaded line starting from the middle point of a loading coil.

n=Frequency of the current in the line.

H=Inductance of each of the loading coils.

Z=Distance between loading coils.

C=Capacity perunitlength of line.

L=Inductance per .unit length of line.

R=Resistance per unit length of line.

capacity, and re-' 19,421, and in Great Britain April 4, 1924) Then if the inductance of the lineapart In practice both less than Hence The imaginary term on the right of Equation (2) represents a condensive reactance of a 20 II series condenser of capacity k= creasing with frequencyaccording to the above relation and of zero reactance.

I have found that the network shown in Figure 1, by choosing suitable values of the elements, can be made to give this required result; the values of the elements being definitely related to the electrical constants of the line. v

The values of the elements are determined in the following way. The impedance of the network of Fig. l

and 1% are always The real part of Z i. e. the resistance com ponent of the impedance of this network is 1 (K 1 2KL pK L (5) 5 which is of. the form- The resistance component of the cable im- 10 pedance from Equation (2) is seen to be P J 'p H0l Cl 4 which is of the form B /1 --A (7 By making 1'=B and choosing K and L of suitable values these' expressions (6) and (7) can be made to have very nearly equal values over-a large range of values of p.

Already we have r=B so that the expres- SIOIIS are equal at zero frequency, and further We can make the two expressions have equal values attwo other frequencies. These freuencies should be near the middle and near t e upper limit of the band of speech frequencies, i. e. about 7000 and 12000 radians per second. t

he procedure is first to determine the values of /1 Ap near the two frequencies mentioned above.f For example for a medi- 3 um'loaded cable the values of /.1Ap are about 0.9 and 0.6.

The problem then is to make m and /1-111) have the values .9 and .6 at' l the same frequenc1es'.- For the frequency at 1 which /1 -Ap has the value .9

A. Thus we must have 1 Thus Thus Thus L, K and r are now all determined in terms of constants of "the cable.

The reactance of'the network of Fig. is from (4) Substituting values from above in the second 7 tance.

Accordingly, if we make L =.9O51' /A the reactance of the network of Fig. 1 will be zero and we shall have a network, which, over the useful telephonic range, gives a nonreactive resistance decreasing with frequency in the desired manner. To completethe balance a series condenser in will be required of the value The balance thus consists of the arrangement of elements as shown in Figure 2, the Value of each element being determined by constants of the cable in accordance with the following table.

#2 oz In practice it may be found that the balance can be improved by altering the values a few per cent from those calculated.

By altering the value of L obviously, the balance can'be made to correspond with a termination at any point in the loading coil from full termination to (.5.45)'=.05 termination; in the latter case of course 13 0.

I claim:

1. Aline balance, the impedance of which over the range of telephonic'frequencies simulates closely the sending end impedance of a periodically loaded cable terminated at half loading coil, the said balance comprising elements including a capacity, an inductance, a second inductance, and a resistance, all arranged in series, and a second capacity connected in parallel with the second inductance and the resistance, the values of the several elements being determined in terms of the constants of the line to be balanced, substantially as described.

2, A. line balance, the impedance of which over the range of telephonic frequencies sim-. ulates closely the sending end impedance of a periodically loaded cable terminated at a fractional coil termination, the said balance comprising elements including a capacity, an

inductance adjusted to the desired fractional loading coil termination, a second inductance, and a resistance, all arranged in series, and a second capacity connected in parallel with the second inductance and the resistance, the values of the several elements first being determined in terms of the constants of the line to be balanced, substantially as described.

' all arranged in. series, and a second capacity connected in parallel with the inductance and the resistance, the values of the several elements being determined in terms of the constants of the line to' be balanced, substantially as described.

4. An artificialline for simulating the sending end impedance of a periodically ponent of the impedance of said line are approximately equal.

6. A line balance for simulating the sending end impedance of a-pcriodically loaded cable terminated at small fractional loading coil termination, comprising elements including a capacity, an inductance adjusted for the desired fractional loading coil termination Within a range of values having .05 and .5 as limiting values, a second inductance, I

and a resistance all arranged in series, and a second capacity connected in parallel with the inductance and the resistance, the elements of the line balance being so proportioned that Within the limits of the band of speech frequencies, the resistance components of the sending end impedance of the cable and of the line balance are approximately equal, and reactance components of the sending end impedance of the cable and of the line balance are approximately equal.

In testimony whereof I aflix my signature. ALBERT CHARLES BARTLETT.

loaded cable terminated at half loading coil the: said line comprising elements including a capacity, an lnductance, a second inductance, and a resistance all arranged in series, a second capaclty connected 1I1- par-.

allel with the second inductance and the resistance, the elements of the artificial line being so proportioned that the resistance component of the sending end impedance of the said periodically loaded cable and the resistance component of the impedance of the artificial line are approximately equal, and the reactance component of the sending end impedance of the said periodically loaded cable, and the reactance component of the impedance of the said artificial line are approximately equal.

5. An artificial line for simulating the sending end impedance of a periodically loaded cable terminated at fractional loading coil termination the said line comprising elements including a capacity, an inductance adjusted to the desired fractional loading coil termination, a second inductance, and a noninductive resistance all arranged in series, and a second capacity connected in parallel with the second inductance and the non-inductive resistance, the elements being so proportioned that Within the limits'of the band of speech frequencies the real component of the senda ing end impedance of the said cable and the real component of-the impedance of the said line are approximately equal, and the imaginary component of the sending end impedance of said cable and the imaginary com- 

